How to Grow Dahlias

Dahlias are mid-to-late season flowers that come in a vast array of colors and forms, from small bedding dahlias to plate-sized blossoms atop 6-foot stems. Native to Mexico and Central America, dahlias boast over 20,000 cultivars and have become darlings of plant breeders and flower shows alike.

Most of us think of dahlias as cut flowers, and while they do make for exceptional arrangements, they also deserve recognition as garden plants. Although they can be a little temperamental and fussy about the weather and their growing conditions, many newer cultivars have been bred to be more reliable and easy-growing.

While often labeled as perennials, dahlias are actually too tender to leave out all winter in all but the warmest climates, where ground temperatures don't dip below 20 degrees Fahrenheit. You either have to dig and store the tubers or grow them as annuals and replace them each year in the spring. Still, they'll grow quickly each season, often reaching maturity and blooming in about eight weeks.

Dahlia Care

For such stunning flowers, dahlias are actually fairly easy to grow. Similar to many bulbed flowers, dahlias are grown from tubers, which can be started indoors in early spring and moved outdoors once the danger of frost has passed. (Likewise, you can wait until the soil has warmed and dried a bit in the spring, then plant them directly outdoors.)

Plant large tubers about 6 to 8 inches deep, while the smaller varieties can be planted 3 to 4 inches deep. Ensure they thrive by choosing a spot with a lot of light and watering regularly. Within a few months, you'll be treated to the best reward—a garden full of stunning, colorful, and oversized blooms.

Dahlia's are ready to be cut once the central flowers are fully opened. Harvest the blooms in the morning, when the plants are full of water, to ensure the longest-lasting flowers. When you get them indoors, plunge the stems into 2 or 3 inches of hot (not boiling) water to seal them. Leave them there until the water cools, then arrange as desired.

Light

In order to flower well, dahlias need full sun, preferably at least six to eight hours a day. In hotter climates (primarily USDA zones 8 and up) they will do better with a bit of shade during the peak afternoon hours, when the sun is especially hot and therefore can risk burning the plants.

Soil

Dahlias like a rich, well-draining soil with plenty of organic matter and a fairly neutral soil pH of around 6.5. If you are planting your dahlia tubers ahead of time in containers, mix in a bit of garden soil for better moisture retention—potting soil can dry out quickly and it's especially important that the tubers stay moist until they have sufficient roots.

Water

Dahlias—especially young plants—do not need a ton of water in order to thrive. In fact, over-watering the plant can actually pose more of a potential issue, as excessive water can cause them to rot. Because the roots of the dahlia are close to the surface of the soil, typical summer rainfall can usually suffice. If you get less than 1 inch of rain in seven days (or are dealing with especially hot temperatures), plan to supplement with additional watering. However, with dahlias, it's important to never let the soil dry out—because they're not deep-rooted, a dry top layer of soil equals a dry plant.

Temperature and Humidity

Timing is especially important when it comes to planting and growing dahlias, as they'll struggle to establish in cold soil. Wait until your final spring frost has passed and the ground temperatures have reached around 60 degrees Fahrenheit (you can get your tubers started indoors ahead of time if you'd like).

Additionally, dahlias like humidity when being stored as tubers, but require no additional humidity in their environment when growing throughout the spring and summer outdoors.

Fertilizer

Dahlias benefit greatly from being treated with fertilizer—the more food they get, the larger roots they will grow, and subsequently the bigger (and more numerous) their flowers will be. Whatever type of fertilizer you choose, look for one with a low nitrogen ratio, and don't fertilize the plants after August. You are going to be digging and storing the tubers soon and you want them ready to go dormant.

Common Pests/Diseases

The pests that bug dahlias the most are pretty typical to other similar seasonal blooms. The biggest issue is definitely slugs, especially while the dahlias are young and small. Earwigs, caterpillars, and thrips can also pose a problem. Additionally, some gardeners say deer love their dahlias and others claim they avoid them. It may depend on what else is available to munch on in your garden, but keep your plants protected just in case.

Dahlias can also be prone to powdery mildew and other fungal diseases. Keep the foliage as dry as possible by allowing for good air circulation. If you notice an infection, treat it with neem oil or another natural solution.